U.S. patent application number 10/357645 was filed with the patent office on 2004-08-05 for location estimation of wireless terminals though pattern matching of signal-strength differentials.
Invention is credited to Spain, David Stevenson.
Application Number | 20040152470 10/357645 |
Document ID | / |
Family ID | 32771037 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040152470 |
Kind Code |
A1 |
Spain, David Stevenson |
August 5, 2004 |
Location estimation of wireless terminals though pattern matching
of signal-strength differentials
Abstract
A method of estimating the location of a wireless terminal is
disclosed that is ideally suited for use with legacy systems. The
illustrative embodiment of the present invention is based on the
observation that the signal strength of a signal from a transmitter
is different at some locations, and, therefore, the location of a
wireless terminal can be estimated by comparing the signal strength
it currently observes against a map or database that correlates
locations to signal strengths. For example, if a particular radio
station is known to be received well at a first location and poorly
at a second location, and a given wireless terminal at an unknown
location is receiving the radio station poorly, it is more likely
that the wireless terminal is at the second location than it is at
the first location.
Inventors: |
Spain, David Stevenson;
(Portola Valley, CA) |
Correspondence
Address: |
DEMONT & BREYER, LLC
SUITE 250
100 COMMONS WAY
HOLMDEL
NJ
07733
US
|
Family ID: |
32771037 |
Appl. No.: |
10/357645 |
Filed: |
February 4, 2003 |
Current U.S.
Class: |
455/456.1 ;
455/67.11 |
Current CPC
Class: |
G01S 5/0252 20130101;
H04W 64/00 20130101 |
Class at
Publication: |
455/456.1 ;
455/067.11 |
International
Class: |
H04Q 007/20; H04B
017/00 |
Claims
What is claimed is:
1. A method for estimating a location of a wireless terminal, said
method comprising: receiving a first signal-strength measurement
for a first signal at said wireless terminal and a second
signal-strength measurement for a second signal at said wireless
terminal; and estimating the location of said wireless terminal
based on the difference of said first signal-strength measurement
minus said second signal-strength measurement.
2. The method of claim 1 wherein said first signal-strength
measurement and said second signal-strength measurement are made at
substantially the same time and at substantially the same
location.
3. The method of claim 1 wherein the task of estimating the
location of said wireless terminal comprises generating a
probability distribution for the location of said wireless terminal
based on the difference of said first signal-strength measurement
minus said second signal-strength measurement.
4. The method of claim 1 further comprising the task of receiving a
third signal-strength measurement for a third signal at said
wireless terminal.
5. The method of claim 4 further comprising the task of estimating
the location of said wireless terminal is based on the difference
of said first signal-strength measurement minus said third
signal-strength measurement.
6. The method of claim 4 wherein said first signal-strength
measurement and said third signal-strength measurement are made at
substantially the same time and at substantially the same
location.
7. The method of claim 4 wherein said third signal-strength
measurement is at the maximum reportable value, and wherein the
task of estimating the location of said wireless terminal comprises
restricting candidate locations to those where said third
signal-strength measurement is expected to be at least equal to the
maximum reportable value.
8. The method of claim 1 wherein the task of estimating the
location of said wireless terminal is performed at said wireless
terminal.
9. The method of claim 1 wherein the task of estimating the
location of said wireless terminal is performed at a location
server.
10. A method for estimating a location of a wireless terminal, said
method comprising: receiving a first signal-strength measurement
for a first signal at said wireless terminal and a second
signal-strength measurement for a second signal at said wireless
terminal; and estimating the location of said wireless terminal by
pattern matching the difference of said first signal-strength
measurement minus said second signal-strength measurement against a
signal-strength database that associates location with tuples of
first signal signal-strength measurements and second signal
signal-strength measurements.
11. The method of claim 10 wherein said first signal-strength
measurement and said second signal-strength measurement are made at
substantially the same time and at substantially the same
location.
12. The method of claim 10 wherein the task of estimating the
location of said wireless terminal comprises generating a
probability distribution for the location of said wireless terminal
is based on the difference of said first signal-strength
measurement minus said second signal-strength measurement.
13. The method of claim 10 further comprising the task of receiving
a third signal-strength measurement for a third signal at said
wireless terminal.
14. The method of claim 13 further comprising the task of
estimating the location of said wireless terminal by pattern
matching the difference of said first signal-strength measurement
minus said third signal-strength measurement against a
signal-strength database that associates location with tuples of
first signal signal-strength measurements and third signal
signal-strength measurements.
15. The method of claim 13 wherein said first signal-strength
measurement and said third signal-strength measurement are made at
substantially the same time and at substantially the same
location.
16. The method of claim 13 wherein said third signal-strength
measurement is at the maximum reportable value, and wherein the
task of estimating the location of said wireless terminal comprises
restricting candidate locations to those where said third
signal-strength measurement is expected to be at least equal to the
maximum reportable value.
17. The method of claim 10 wherein the task of estimating the
location of said wireless terminal is performed at said wireless
terminal.
18. The method of claim 10 wherein the task of estimating the
location of said wireless terminal is performed at a location
server.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The underlying concepts, but not necessarily the
nomenclature, of these applications are incorporated by
reference:
[0002] (i) U.S. Pat. No. 6,269,246, issued 31 Jul. 2001;
[0003] (ii) U.S. patent application Ser. No. 09/532,418, filed 22
Mar. 2000;
[0004] (iii) U.S. patent application Ser. No. 10/128,128, filed 22
Apr. 2002; and
[0005] (iv) U.S. patent application Ser. No. 10/299,398, filed 18
Nov. 2002.
FIELD OF THE INVENTION
[0006] The present invention relates to telecommunications in
general, and, more particularly, to a technique for estimating the
location of a wireless terminal.
BACKGROUND OF THE INVENTION
[0007] FIG. 1 depicts a map of a geographic region that is serviced
by a wireless telecommunications system, which system provides
wireless telecommunications service to wireless terminals (e.g.,
wireless terminal 101) within the region. The heart of the
telecommunications system is wireless switching center 110, which
might also be known as a mobile switching center ("MSC") or a
mobile telephone switching office ("MTSO").
[0008] Typically, wireless switching center 111 is connected to a
plurality of base stations (e.g., base stations 102-A through
102-C), which are dispersed throughout the geographic area serviced
by the system. As is well known to those skilled in the art,
wireless switching center 111 is responsible for, among other
things, establishing and maintaining calls between wireless
terminals and between a wireless terminal and a wireline terminal
(which is connected to the system via the local and/or
long-distance networks and which are not shown in FIG. 1).
[0009] The salient advantage of wireless telecommunications over
wireline telecommunications is the mobility that is afforded to the
users of the wireless telecommunications system. On the other hand,
the salient disadvantage of wireless telecommunications lies in
that fact that because the user is mobile, an interested party
might not be able to readily ascertain the location of the
user.
[0010] Such interested parties might include both the user of the
wireless terminal and remote parties. There are a variety of
reasons why the user of a wireless terminal might be interested in
knowing his or her own location. For example, the user might be
interested in telling a remote party where he or she is.
[0011] There are a variety of reasons why a remote party might be
interested in knowing the location of the user. For example, the
recipient of a 911 emergency call from a wireless terminal might be
interested in knowing the location of the wireless terminal so that
emergency services vehicles can be dispatched to that location.
[0012] There are many techniques in the prior art for estimating
the location of a wireless terminal.
[0013] In accordance with one technique, the location of a wireless
terminal is estimated to be at the center of the cell in which the
wireless terminal is located. This technique is advantageous in
that it does not require that additional hardware be added to the
wireless terminal or to the wireless telecommunications system, and
this means that the first technique can be inexpensively
implemented in legacy systems. The first technique is only
accurate, however, to a few kilometers, and, therefore, it is
generally not acceptable for applications (e.g., emergency services
dispatch, etc.) that require higher accuracy.
[0014] In accordance with a second technique, the location of a
wireless terminal is estimated by triangulating the angle of
arrival or the time of arrival of the signals transmitted by the
wireless terminal to be located at various receivers. This
technique is accurate to within a few hundreds of meters and is
advantageous in that it can be used with legacy wireless terminals.
It is disadvantageous, however, in that it generally requires that
hardware be added to the telecommunication system's base stations,
and this is very expensive.
[0015] In accordance with a third technique, the location of a
wireless terminal is estimated by a radio navigation unit, such as
a Global Positioning System receiver, that is incorporated into the
wireless terminal. This technique is accurate to within tens of
meters and is advantageous in that it does not require that
additional hardware be added to the telecommunication system's
infrastructure. The third technique is disadvantageous, however, in
that it cannot be used with legacy wireless terminals that do not
comprise a radio navigation unit.
[0016] Therefore, the need exists for a technique for estimating
the location of a wireless terminal with higher resolution than the
first technique and that can be inexpensively implemented in legacy
systems.
SUMMARY OF THE INVENTION
[0017] The present invention enables the estimation of the location
of a wireless terminal without the addition of hardware to either
the wireless terminal or to the telecommunication system's base
stations. Some embodiments of the present invention are, therefore,
ideally suited for use with legacy systems.
[0018] The illustrative embodiment of the present invention is
based on the observation that the signal strength of a signal from
a transmitter is different at some locations, and, therefore, the
location of a wireless terminal can be estimated by comparing the
signal strength it currently observes against a map or database
that correlates locations to signal strengths. For example, if a
particular radio station is known to be received well at a first
location and poorly at a second location, and a given wireless
terminal at an unknown location is receiving the radio station
poorly, it is more likely that the wireless terminal is at the
second location than it is at the first location.
[0019] When this same principal is applied to multiple transmitters
and multiple signals, the location of a wireless terminal can be
discriminated among locations where the signal strength of one
signal is the same. A simple example illustrates this point. A
first radio station, Radio Station A, can be received well at
Location 1 and Location 2, but poorly at Location 3 and Location 4,
and a second radio station, Radio Station B, can be received well
at Location 1 and Location 3, but poorly at Location 2 and Location
4. This information is summarized in the table below and forms the
basis for a map or database that correlates location to signal
strength.
1TABLE 1 Illustrative Signal Strength Database (Absolute Reception)
Radio Station A Radio Station B Location 1 Good Reception Good
Reception Location 2 Good Reception Poor Reception Location 3 Poor
Reception Good Reception Location 4 Poor Reception Poor
Reception
[0020] If a given wireless terminal at an unknown location can
receive Radio Station A poorly and Radio Station B well, it is more
likely that the wireless terminal is at Location 3 than it is at
either Location 1, 2, or 4.
[0021] Furthermore, the location of a wireless terminal can be
estimated with a more-than-acceptable degree of accuracy when the
signal strength of each signal at each location is quantified. For
example, if a particular radio station is known to be received in
one location with a strength of -50 dBm, at a second location with
a strength of -53 dBm, and at a third location with a strength of
-55 dBm, then the reception of the signal with a strength of -56
dBm suggests that the wireless terminal is more likely at the third
location than at either the first or second location.
[0022] These principals largely assume, however, that the equipment
used to measure the signal strength of the signals is uniformly
calibrated, and this, unfortunately, is not always the case is the
real world. Many factors, including the make and model of a
wireless terminal, the condition of its antenna, the state of its
battery, and whether the terminal is inside a vehicle or not can
introduce measurement biases that can cause errors in estimating
the location of a wireless terminal.
[0023] The illustrative embodiment ameliorates the effects of these
biases by pattern matching not the observed signal strengths to the
predicted signal strengths themselves, but by pattern matching the
pair-wise differentials of the observed signals strengths to the
pair-wise differentials of the predicted signal strengths. A simple
example illustrates this nuance. A first radio station, Radio
Station A, can be received at -56 dBm at Location 1, -42 dBm at
Location 2, -63 dBm at Location 3, and -61 dBm at Location 4, and a
second radio station, Radio Station B, can be received at -63 dBm
at Location 1, -56 dBm at Location 2, -65 dBm at Location 3, and
-52 dBm at Location 4. This information is summarized in the table
below and forms the basis for a map or database that correlates
location to signal strength.
2TABLE 2 Illustrative Signal Strength Database (Differential
Reception) Radio Radio Station A Station B Difference Location 1
-56 dBm -63 dBm -7 dBm Location 2 -42 dBm -56 dBm -14 dBm Location
3 -63 dBm -65 dBm -2 dBm Location 4 -61 dBm -52 dBm 9 dBm
[0024] If a given wireless terminal with a broken antenna and at an
unknown location receives Radio Station A at -47 dBm and Radio
Station B at -61 dBm, then it registers Radio Station A as 14 dBm
stronger than Radio Station B. This suggests that the wireless
terminal is more likely to be at Location 2 than it is at Location
1, 3, or 4.
[0025] A disadvantage of this approach is that the common bias is
eliminated at the expense of (1) doubling the variance of the
random measurement noise, and (b) by reducing the number of data
points to match by one. Furthermore, the pair-wise subtraction
introduces correlation into the relative signal strength
measurement errors (i.e., all of the data points to be matched are
statistically correlated). This correlation must be accounted for
in calculating the likelihood of the measurement report.
[0026] The illustrative embodiment comprises: receiving a first
signal-strength measurement for a first signal at the wireless
terminal and a second signal-strength measurement for a second
signal at the wireless terminal; and estimating the location of the
wireless terminal based on the difference of the first
signal-strength measurement minus the second signal-strength
measurement.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 depicts a map of a portion of a wireless
telecommunications system in the prior art.
[0028] FIG. 2 depicts a map of the illustrative embodiment of the
present invention.
[0029] FIG. 3 depicts a block diagram of the salient components of
location system 212.
[0030] FIG. 4 depicts a broad overview of the salient operations
performed by the illustrative embodiment in ascertaining the
location of wireless terminal 201 in geographic region 200.
[0031] FIG. 5 depicts a flowchart of the salient operations
performed in operation 401.
[0032] FIG. 6 depicts a map of how geographic region 200 is
partitioned into 500 locations in accordance with the illustrative
embodiment of the present invention.
[0033] FIG. 7a depicts a graph that shows that the signal-strength
of an electromagnetic signal decreases, in general, as a function
of the distance from the transmitter and in an environment with no
radio frequency obstacles.
[0034] FIG. 7b depicts a graph that shows that the signal-strength
of an electromagnetic signal decreases, in general, as a function
of the distance from the transmitter and in an environment with two
radio frequency obstacles.
[0035] FIG. 8 depicts a map of the signal-strength measurements of
the signal radiated from base station 202-1 at each location in
geographic region 200.
[0036] FIG. 9 depicts a map of the signal-strength measurements of
the signal radiated from base station 202-2 at each location in
geographic region 200.
[0037] FIG. 10 depicts a map of the signal-strength measurements of
the signal radiated from base station 202-3 at each location in
geographic region 200.
[0038] FIG. 11 depicts a flowchart of the salient operations
performed in operation
[0039] FIG. 12 depicts a flowchart of the salient operations
performed in operation 403.
DETAILED DESCRIPTION
[0040] FIG. 2 depicts a map of the illustrative embodiment of the
present invention, which comprises: wireless switching center 211,
location system 212, base stations 202-1, 202-2, and 202-3, and
wireless terminal 201, which are interconnected as shown. The
illustrative embodiment provides wireless telecommunications
service to most of geographic region 200, in well-known fashion,
and is also capable of estimating the location of wireless terminal
201 within geographic region 200.
[0041] The illustrative embodiment operates in accordance with the
Global System for Mobile Communications (formerly known as the
Groupe Speciale Mobile), which is ubiquitously known as "GSM."
After reading this disclosure, however, it will be clear to those
skilled in the art how to make and use embodiments of the present
invention that operate in accordance with other protocols, such as
the Universal Mobile Telephone System ("UMTS"), CDMA-2000, and
IS-136 TDMA.
[0042] Wireless switching center 211 is a switching center as is
well-known to those skilled in the art in most respects but is
different in that it is capable of communicating with location
system 212 in the manner described below. After reading this
disclosure, it will be clear to those skilled in the art how to
make and use wireless switching center 211.
[0043] Base stations 202-1, 202-2, and 202-3 are well-known to
those skilled in the art and communicate with wireless switching
center 211 through cables and other equipment (e.g., base station
controllers, etc.) that are not shown in FIG. 2. Although the
illustrative embodiment comprises three base stations, it will be
clear to those skilled in the art how to make and use embodiments
of the present invention that comprise any number of base
stations.
[0044] Wireless terminal 201 is a standard GSM wireless terminal,
as is currently manufactured and used throughout the world.
Wireless terminal 201 is equipped, in well-known fashion, with the
hardware and software necessary to measure and report to wireless
switching center 211 on the signal-strength of the control and
traffic channels from base stations 202-1, 202-2, and 202-3.
[0045] A GSM wireless terminal, such as wireless terminal 201, is
capable of reporting the signal strength of a signal as one of 64
levels between -47 dBm and -110 dBm. Any signal stronger than -47
dBm is reported as -47 dBm, and any signal weaker than -110 dBm is
reported as -110 dBm.
[0046] In accordance with the illustrative embodiment of the
present invention all of the specific portions of the radio
frequency spectrum fall within the same band that wireless terminal
201 uses to communicate with base stations 202-1, 202-2, and 202-3.
In some alternative embodiments of the present invention, however,
some or all of the specific portions of the radio frequency
spectrum are outside the band that wireless terminal 201 uses to
communicate with base stations 202-1, 202-2, and 202-3. In any
case, it will be clear to those skilled in the art how to make and
use wireless terminal 201.
[0047] Location system 212 is a computer system that is capable of
estimating the location of wireless terminal 201, as described in
detail below. Although the illustrative embodiment depicts location
system 212 as estimating the location of only one wireless
terminal, it will be clear to those skilled in the art that
location system 212 is capable of estimating the location of any
number of wireless terminals serviced by wireless switching center
211.
[0048] Although location system 212 is depicted in FIG. 2 as a
distinct entity from wireless switching center 211, this is done
principally to highlight the distinction between the functions
performed by wireless switching center 211 and the functions
performed by location system 212. In other words, it will be clear
to those skilled in the art how to make and use embodiments of the
present invention in which location system 212 resides within or
without wireless switching center 211.
[0049] Although--again for pedagogical purposes--wireless switching
center 211, location system 212, and base stations 202-1, 202-2,
and 202-3 are depicted in FIG. 2 as being within geographic region
200, this is not necessarily so, and it will be clear to those
skilled in the art how to make and use embodiments of the present
invention in which some or all of these pieces of equipment are not
within the region of location estimation.
[0050] FIG. 3 depicts a block diagram of the salient components of
location system 212, which comprises: processor 301 and
signal-strength database 302, which are interconnected as
shown.
[0051] Processor 301 is a general-purpose processor as is
well-known in the art that is capable of performing the operations
described below and with respect to FIG. 4.
[0052] Signal-strength database 302 is a non-volatile memory that
stores signal-strength measurements as described below and with
respect to FIG. 4.
[0053] Overview--FIG. 4 depicts a broad overview of the salient
operations performed by the illustrative embodiment in ascertaining
the location of wireless terminal 201 in geographic region 200. In
summary, the tasks performed by the illustrative embodiment can be
grouped for ease of understanding into four operations:
[0054] i. the population of signal-strength database 302,
[0055] ii. the receipt of signal-strength measurements from
wireless terminal 201,
[0056] iii. the estimation of the location of wireless terminal
201, and
[0057] iv. the use of the estimated location of wireless terminal
201.
[0058] The details of each of these operations are described
briefly below and in detail afterwards with respect to FIG. 5
though 12.
[0059] At operation 401, signal-strength database 302 is populated
with data that associates each location within geographic region
200 with a tuple of signal-strength measurements for that location.
Operation 401 is generally complex and potentially expensive, and
it is, therefore, preferably performed only occasionally. The
details of operation 401 are described in detail below and with
respect to FIG. 5.
[0060] At operation 402, location system 212 receives a tuple of
signal-strength measurements from wireless terminal 201. In
accordance with the illustrative embodiment, wireless terminal 201
periodically or sporadically provides a tuple of signal-strength
measurements to wireless switching center 211 in well-known
fashion, and the measurements are forwarded to location system 212.
The details of operation 402 are described in detail below and with
respect to FIG. 11.
[0061] At operation 403, location system 212 estimates the location
of wireless terminal 201 based on:
[0062] (i) the tuple of signal-strength measurements received in
operation 402, and
[0063] (ii) the tuples of signal-strength measurements within
signal-strength database 302. The details of operation 403 are
described in detail below and with respect to FIG. 12.
[0064] At operation 404, location system 212 transmits the location
estimated in operation 403 to an entity (not shown) for use in an
application. It is well known to those skilled in the art how to
use the estimated location of a wireless terminal in an
application.
[0065] At this point, each of these four operations is described in
detail.
[0066] Population of Signal-Strength Database 302--FIG. 5 depicts a
flowchart of the salient operations performed in operation 401.
[0067] At task 501, geographic region 200 is partitioned into a
plurality of tessallated locations. Geographic region 200 is
rectangular and comprises 5,525 square arc-seconds, which near the
equator equals almost 5 square kilometers. After reading this
specification, it will be clear to those skilled in the art how to
make and use embodiments of the present invention that operate with
geographic regions of any size and shape.
[0068] In accordance with the illustrative embodiment of the
present invention, geographic region 200 is partitioned into a grid
of 221 square locations that are designated location x.sub.1,
y.sub.1 through location x.sub.17, y.sub.13. The number of
locations into which geographic location 200 is partitioned is
arbitrary, subject to the considerations described below. In
accordance with the illustrative embodiment, each location is an
area of approximately 5 arc-seconds in length by 5 arc-seconds in
height. Five arc-seconds near the equator equals approximately 150
meters.
[0069] The size of the locations defines the highest resolution
with which the illustrative embodiment can locate a wireless
terminal. In other words, the illustrative embodiment can only
estimate the location of a wireless terminal to within one location
(i.e., 5 by 5 arc-seconds in the illustrative embodiment). If
greater resolution is desired, for example 1 arc-second resolution,
then geographic region 200 would need to be partitioned into 1
arc-second by 1 arc-second locations. If geographic region 200 were
partitioned into 1 arc-second by 1 arc-second locations, there
would be 5,525 squares, which is considerably more than the 221
used in the illustrative embodiment. Although the ostensibly higher
resolution of 1 arc-second versus 5 arc-seconds is advantageous,
there are considerable disadvantages to a large number of
locations.
[0070] The number of locations to partition geographic region 200
into is based on three factors. First, as the size of each location
goes down, the resolution of the embodiment increases. Second, as
the size of each location goes down, the number of locations in a
region increases, and, consequently, the computational complexity
of operation 403 increases quickly. Third, each location must be
large enough so that it has (at least slightly) different
signal-strength characteristics than its adjacent areas. This is
because the illustrative embodiment might--but won't
necessarily--have difficulty distinguishing between adjacent
location that have the same signal-strength characteristics. It
will be clear to those skilled in the art how to consider these
three factors when deciding how to partition a geographic
region.
[0071] At task 502, the signal-strength measurements for a signal
from each base station are determined at each location in
geographic region. In accordance with the illustrative embodiment,
the signal used from each base station is the control channel
because it is broadcast at a constant power and because wireless
terminal 201 can distinguish it from every other control channel,
if it can decode its BSIC (for GSM networks).
[0072] Because there are three base stations in the illustrative
embodiment, each with one control channel, a tuple of three
signal-strength measurements at each location must be
determined.
[0073] In general, the signal-strength of an electromagnetic signal
decreases as a function of the distance from the transmitter, as is
depicted in FIG. 7a, but the topography of the region and the
presence of buildings, trees, and other radio-frequency obstacles
severely alters this generalization, as is depicted in FIG. 7b.
[0074] In accordance with the illustrative embodiment, the tuple of
three signal-strength measurements for each location are determined
through a combination of:
[0075] (i) a theoretical radio-frequency propagation model, and
[0076] (ii) empirical signal-strength measurements.
[0077] It will be clear to those skilled in the art how to
accomplish this.
[0078] For example, one well-known modeling for outdoor
radio-frequency signal propagation is adapted from the power-law
decay model. The power-law decay model assumes that the base
station's antenna is high above the ground and that there is
line-of-sight propagation to the wireless terminal. In this case,
the mean signal-strength, P, received at the wireless terminal
decays in inverse proportion to the square of the distance from the
transmitter, 1 P 1 r 2 , ( Eq . 1 )
[0079] up to some break-point. Beyond that breakpoint, the mean
power at the wireless terminal decays in inverse proportion to the
fourth power of the distance from the transmitter: 2 P 1 r 4 ( Eq .
2 )
[0080] The location of the break-point is determined through
empirical signal-strength measurements as the location at which the
ground bounce signal interferes with the line-of-sight signal.
[0081] In accordance with another well-known model, the
signal-strength measurements at each location are determined by
taking empirical measurements at various locations and by
interpolating for the locations in between the sampled locations.
This method is advantageous in that it does not require many
empirical measurements to be made, but it is less accurate than
taking measurements at every location.
[0082] It will be clear to those skilled in the art how to
determine the signal-strength measurements for each location in the
geographic region whether through:
[0083] (i) theoretical radio-frequency propagation models, or
[0084] (ii) empirical signal-strength measurements, or
[0085] (iii) any combination of i and ii.
[0086] In accordance with the illustrative embodiment, FIG. 8
depicts the signal-strength of the signal from base station 202-1
(hereinafter referred to as "Signal 1") at each location in
geographic region 200. In general, Signal 1 is stronger near base
station 202-1 and weaker far away from base station 202-1.
[0087] In accordance with the illustrative embodiment, FIG. 9
depicts the signal-strength of the signal from base station 202-2
(hereinafter referred to as "Signal 2") at each location in
geographic region 200. Like Signal 1, Signal 2 is stronger near
base station 202-2 and weaker far away from base station 202-2.
[0088] In accordance with the illustrative embodiment, FIG. 10
depicts the signal-strength of the signal from base station 202-3
(hereinafter referred to as "Signal 3") at each location in
geographic region 200. Like Signals 1 and 2, Signal 3 is stronger
near base station 202-3 and weaker far away from base station
202-3.
[0089] When the signal-strength tuples for each location in
geographic region 200 have been determined, they are stored in
signal-strength database in a data structure that associates each
location with the tuple for that location. The data structure is
then stored in signal-strength database 302.
3TABLE 3 Signal-Strength Database Signal-Strength Tuple Strength of
Strength of Strength of Location Signal 1 Signal 2 Signal 3
x.sub.1, y.sub.1 -115 -115 -115 x.sub.2, y.sub.1 -115 -115 -111 . .
. . . . . . . . . . X.sub.7, y.sub.7 -45 -51 -49 X.sub.8, y.sub.7
-46 -52 -55 X.sub.9, y.sub.7 -50 -49 -62 . . . . . . . . . . . .
X.sub.16, y.sub.13 -115 -96 -115 X.sub.17, y.sub.13 -115 -105
-115
[0090] Table 3 depicts a portion of an illustrative data structure
for associating each location with the signal-strength tuple for
that location.
[0091] The three signal-strength measurements in a row of table 1
constitute a "tuple" or non-empty set of ordered elements. For
example, the signal-strength tuple at Location x.sub.7, y.sub.7 are
the 3-tuple {-45, -51, -49}. In general, the illustrative
embodiment of the present invention estimates the location of a
wireless terminal by pattern matching the signal-strength
measurements by the wireless terminal at a location against the
signal-strength tuples in signal-strength database 302. This
process is described in detail below and with respect to operation
402.
[0092] From task 502, control passes to operation 402 in FIG.
4.
[0093] Receipt of Signal-Strength Measurements from Wireless
Terminal 201--FIG. 11 depicts a flowchart of the salient operations
performed in operation 402.
[0094] At task 1101, wireless switching center 211 determines, in
well-known fashion, which signals wireless terminal 201 might or
might not be able to receive. In accordance with the illustrative
embodiment, wireless switching center 211 determines that wireless
terminal 201 might be able to receive Signal 1, Signal 2, and
Signal 3.
[0095] At task 1102, wireless switching center 211 directs wireless
terminal 201, in well-known fashion, to attempt to receive the
signals it might be able to receive and to report back a
signal-strength value for the strongest (up to) 3 signals that
wireless terminal 201 is able to receive and distinguish. In
accordance with the illustrative embodiment, wireless switching
center 211 directs wireless terminal 211 to monitor Signal 1,
Signal 2, and Signal 3 and to report on all of these signals.
[0096] At task 1103, location system 212 receives, via wireless
switching center 211, a report from wireless terminal 201 on the
signals that it was directed to monitor in task 602.
[0097] As described above, wireless terminal 201 is incapable of
reporting a signal whose signal-strength is equal to -46 dBm or
higher, and, therefore, when wireless terminal 201 attempts to
report a signal whose signal-strength is -46 dBm or higher,
wireless terminal 201 simply reports a signal-strength value of -47
dBm for that signal. The significance of this insight is that a
reported signal-strength value of -47 dBm might not accurately
reflect the magnitude of that signal's strength at that location.
To further illustrate the significance of this insight and its
effect on the design of the illustrative embodiment, this
specification shall describe in detail how two different
signal-strength reports are processed by the illustrative
embodiment. In accordance with the first report, the
signal-strength of all three signals is low enough so that wireless
terminal 201 can report the actual strength of the signals. In
accordance with the first report, Signal 1=-98, Signal 2=-64, and
Signal 3=-51. In accordance with the second report, Signal 1=-98,
Signal 2=-64, and Signal 3=-50. How these two types of reports are
handled is described below and with respect to FIG. 12.
[0098] It will be clear to those skilled in the art how to make and
use embodiments of the present invention that perform operation
402. From task 603, control passes to operation 403 in FIG. 4.
[0099] Estimation of the Location of Wireless Terminal 201--FIG. 12
depicts a flowchart of the salient operations performed in
operation 403. For pedagogical purposes, operation 403 as depicted
in FIG. 12 is described three times. First, operation 403 is
described in the abstract with a focus on describing its underlying
theory. Next, operation 403 is described as it is applied to the
first report, and finally, operation 403 is described as it is
applied to the second report.
[0100] Estimation in General--Task 1201 begins with 211
(17.times.13=211) candidate locations that must be considered as
the location for wireless terminal 201, and, therefore, 211
signal-strength tuples (i.e., the 211 tuples in signal-strength
database 302) that must be processed. Tasks 1202 through 1205 can
be computationally intense, and the computational burden increases
with the number of candidate locations that must be considered.
Therefore, location system 212 attempts, at task 1201, to reduce
the number of candidate locations that must be processed in tasks
1202 through 1205.
[0101] To reduce the number of candidate locations that must be
processed in tasks 1202 through 1205, location system 212 uses the
following observation. When a signal is reported with a maximum
signal-strength (i.e., "-47" in the illustrative embodiment),
location system 212 can reasonably eliminate from consideration as
a candidate location every location where the signal-strength
measurement for that signal is below the maximum (minus a factor
for measurement errors and systematic bias). In other words, when a
signal is reported with a maximum signal-strength, location system
212 can restrict consideration in tasks 1202 through 1205 to those
candidate locations where signal-strength database 302 predicts the
signal-strength to be greater than or equal to the maximum
reportable value (minus a factor for measurement errors and
systematic bias). In accordance with the illustrative embodiment,
the factor for measurement errors and systematic bias is 3 dBm,
and, therefore when a signal is reported with -47, location system
212 can restrict consideration in tasks 1202 through 1205 to those
candidate locations where signal-strength database 302 predicts the
signal-strength to be greater than or equal to -50 dBm. It will be
clear to those skilled in the art how to determine and use other
factors for measurement errors and systemic bias.
[0102] At task 1202, location system 212 computes the
signal-strength differentials for those reported values that are
not at the maximum signal-strength. In particular, for n reported
signals that are not at the maximum signal-strength, n-1
signal-strength differentials are computed where:
.DELTA.S.sub.k=S.sub.k-S.sub.1 (Eq. 1)
[0103] for k=2, 3, . . . n, wherein .DELTA.S.sub.k is the kth
signal-strength differential, S.sub.k is the reported
signal-strength of Signal k, and S.sub.1 is the reported
signal-strength of Signal 1. When m of the reported signals is at
the maximum signal-strength (i.e., -47 dBm), then
n-m-1 (Eq. 2)
[0104] pair-wise differentials for the remaining n-m signals are
computed, in well-known fashion. At the end of task 1202, location
system 212 will have computed n-m-1 pair-wise differentials,
.DELTA.S.sub.2 through .DELTA.S.sub.n-m.
[0105] At task 1203, location system 212 computes the
signal-strength differentials for only those locations that were
not eliminated from consideration in task 1201. Furthermore,
location system 212 only computes the signal-strength differentials
corresponding to the differentials computed in task 1202; the idea,
of course, being to ensure that "apples are compared with apples."
In particular, for n reported signals that are not at the maximum
signal-strength, n-1 signal-strength differentials are computed
where:
.DELTA.R.sub.k,x,y=R.sub.k,x,y-R.sub.1,x,y (Eq. 3)
[0106] for k=2, 3, . . . n, wherein .DELTA.R.sub.k,x,y is the kth
signal-strength differential for location x,y, R.sub.k,x,y is the
signal-strength of Signal k at location x,y in signal-strength
database 302, and R.sub.1,x,y is the reported signal-strength of
Signal 1 at location x,y in signal-strength database 302.
[0107] At the end of task 1203, location system 212 will have
computed n-m-1 pair-wise differentials, .DELTA.R.sub.2,x,y through
.DELTA.R.sub.n-m,x,y, corresponding to the pair-wise differentials
computed in task 1203, for all the candidate locations.
[0108] At task 1204, location system 212 compares the
signal-strength differentials computed in task 1202, .DELTA.S.sub.2
through .DELTA.S.sub.n-m, to the signal-strength differentials in
task 1203, .DELTA.R.sub.2,x,y through .DELTA.R.sub.n-m,x,y, to
generate a probability distribution that indicates the goodness of
fit between the signal-strength differentials computed from the
values received in operation 402 to the signal-strength
differentials computed from the tuples in signal-strength database
302. To accomplish this, the Euclidean norm at each of the i
candidate locations is computed for the signal-strength
differentials computed from the values received in operation 402
and each of the signal-strength differentials computed from the
tuples in signal-strength database 302. This is described in
Equation 4.
v.sub.x,y={square root}{square root over
(.SIGMA..sub.2.sup.n(.DELTA.R.sub- .k,x,y-.DELTA.S.sub.k).sup.2)}
(Eq. 4)
[0109] wherein v.sub.x,y is the Euclidean norm between the
signal-strength tuple for location x,y in signal-strength database
302 in comparison to the signal-strength differentials computed
from the values received in operation 402.
[0110] Next, the Euclidean norms computed in Equation 4 are turned
into un-normalized probabilities by Equation 5: 3 p x , y = - v x ,
y 2 2 ( Eq . 5 )
[0111] where .tau..sup.2 represents the amount of uncertainty in
both .DELTA.S.sub.k and .DELTA.R.sub.k,x,y.
[0112] And finally, the values of P.sub.x,y are normalized to
generate the probability distribution for the location of wireless
terminal 201 in geographic region 200.
[0113] At task 1205, location system 212 estimates the location of
wireless terminal 201 based on the probability distribution
generated in task 1204. In accordance with the illustrative
embodiment, location system 212 estimates the location of wireless
terminal based on the geometric mean of the probability
distribution generated in task 1204. After reading this
specification, however, it will be clear to those skilled in the
art how to make and use embodiments of the present invention that
estimate the location of wireless terminal 201 based on another
function of the probability distribution generated in task 1204,
such as the maximum likelihood function.
[0114] From task 1205, control passes to operation 404 in FIG.
4.
[0115] Estimation as Applied to First Report (Signal 1=-98, Signal
2=-64, and Signal 3=-51)--At task 1201, location system 212 cannot
eliminate any candidate locations from consideration based on the
fact that none of the reported signals is at the maximum reportable
value minus the factor for measurement errors and systematic bias
(i.e., 3 dBm). In other words, location system 212 cannot eliminate
any candidate signal from consideration because all of the signals
are at -51 dBm or less. Therefore, location system 212 must
consider all 221 candidate locations in tasks 1202 through
1205.
[0116] At task 1202, location system 212 computes two (2)
signal-strength differentials for the first report in which
R.sub.1=Signal 1=-98, R.sub.2=Signal 2=-64, and R.sub.3 Signal
3=-43. In particular, .DELTA.R.sub.2 and .DELTA.R.sub.3 are
computed as depicted in Table 4.
4TABLE 4 Signal-strength Differentials for Signal 1 = -98, Signal 2
= -64, and Signal 3 = -43 k .DELTA.R.sub.k R.sub.k - R.sub.1 2 34
-64 - (-98) 3 47 -51 - (-98)
[0117] At task 1203, location system 212 computes two (2)
signal-strength differentials for each of the 221 locations in
signal-strength database 302, as depicted in Table 5.
5TABLE 5 Signal-strength Differentials for Each Tuple in
Signal-Strength Database 302 Location .DELTA.S.sub.2,x,y
.DELTA.S.sub.3,x,y x1, y1 -110 - (-110) = 0 -110 - (-110) = 0 x2,
y1 -110 - (-110) = 0 -111 - (-110) = -1 x3, y1 -110 - (-110) = 0
-97 - (-110) = 3 . . . . . . . . . x16, y13 -96 - (-110) = 14 -110
- (-110) = 0 x17, y13 -105 - (-110) = 5 -110 - (-110) = 0
[0118] At task 1204, location system 212 first computes the
Euclidean norm between the signal-strength differentials in Table 2
against the signal-strength differentials for each location in
Table 3 to produce the norms shown in Table 6.
6TABLE 6 Euclidean Norms for Each Location (First Report) Location
Vx,y x1, y1 64.66 x2, y1 63.81 x3, y1 62.13 . . . . . . x16, y13
58.52 x17, y13 62.18
[0119] Next, the Euclidean norms in Table 6 are converted to
unnormalized probabilities, as described above, and then the
unnormalized probabilities are normalized, in well-known fashion,
to produce the probability distribution of the location of wireless
terminal 201 at each of the 211 locations in geographic region
200.
[0120] Estimation As Applied to Second Report (Signal 1=-98, Signal
2=-64, and Signal 3=-50)--At task 1201, location system 212 can
perfunctorily eliminate most of the candidate locations from
consideration because the reported signal-strength of one of the
reported signals--Signal 3=-50 dBm--is greater than the maximum
reported value (-47 dBm) minus the factor for measurement errors
and systematic bias (3 dBm). In other words, location system 212
can eliminate from consideration any candidate location in which
S.sub.3 is not at least -50 dBm. Therefore, location system 212 can
restrict consideration in tasks 1202 through 1205 to those
locations in signal-strength database 302 in which Signal 3 is
predicted to be -50 dBm or greater. As can be seen in FIG. 10,
there are only 14 locations (x8, y4; x9, y4; x10, y4; x7, y5; x8,
y5; x9, y5; x10, y5; x7, y6; x8, y6; x9, y6; x10, y6; x7, y7; x8,
y7; x9, y7) at which Signal 3 is predicted to be -50 dBm or
stronger, and, therefore, location system 212 need only perform
tasks 1202 through 1205, in the above-described fashion, on those
14 locations. By reducing the number of candidate locations that
need to be processed from 221 to 14, task 1201 has greatly reduced
the computational complexity of operation 403.
[0121] It is to be understood that the above-described embodiments
are merely illustrative of the present invention and that many
variations of the above-described embodiments can be devised by
those skilled in the art without departing from the scope of the
invention. It is therefore intended that such variations be
included within the scope of the following claims and their
equivalents.
* * * * *